In the field of integrated circuits (ICs), photolithography is used to transfer patterns, i.e. images, from a mask containing, circuit-design information to thin films on the surface of a substrate, e.g. Si wafer. The pattern transfer is accomplished with a photoresist (e.g., an ultraviolet light-sensitive organic polymer). In a typical image transfer process, a substrate that is coated with a photoresist is illuminated through a mask and the mask pattern is transferred to the photoresist by chemical developers. Further pattern transfer is accomplished using a chemical etchant. In current technologies, this masking process usually is repeated multiple times in the fabrication of an integrated circuit.
FIG. 1 illustrates a typical photolithographic processing (fab) environment comprising a mask 110, a stepper device 120 with lens 130 through which the exposure energy 140 is focused on a wafer 150 coated with a photoresist 160.
It is well known in the field of photolithography that Critical Dimension may change as a result of either effective exposure or tool focus. Prior art tool focus control monitoring and feed back is achieved by imaging a specific image design on non-patterned test wafers with individual exposure fields stepped through a range of z focus. It is a well known practice to define the center of the focus step range yielding the best image CD stability as the nominal tool focus. This image stability has, in the past, either been subjectively determined by reading focus “dots” under a microscope or by measuring a change in 2 dimensional line width on a pattern such as a photoresist chevron or measuring the foreshortening of a series of lines and spaces. The result of this prior art method is that an offset from this nominal tool focus to product focus of a specific masking level must be controlled by implementing a table of run rules, as shown in FIG. 13. In the table of run rules, the focus 1370, default exposure 1360, are related to the tool 1310, technology 1320, wafer part number 1330 and process 1340. Under the prior art method, the optimal product focus offset is a fixed offset. It is well known in the industry, however, that for a given photolithographic exposure tool, there is variability of nominal tool focus. Thus, optimal product focus offset is variable as a result of the inherent tool focus variation. This-prior art technique has thus required that two dimensional critical dimension be monitored on product masking levels so that shifts in CD are compensated for by feeding back a change in dose to the exposure tool. As discussed below, the related art has not solved the problem of real time variation in optimal product focus offset due to photolithographic exposure tool nominal focus instability.
A related art technique for across field dimensional control in a microlithography tool is described in Borodovsky (U.S. Pat. No. 6,021,009) which is directed to a lithography tool adjustment method through which light intensity is varied to reduce line width variations. This process however, does not measure critical features to determine a z focus parameter, nor does it differentiate between feature error components due to focus shift and other process factors.
Another related art method is described in Lai (U.S. Pat. No. 6,190,810 B1) which utilizes a single spot laser focusing system in which the laser light spot is always positioned between a series of registration marks. However, Lai does not teach the use of critical design features, i.e. line edge width (EW) and contact hole profile, for z focus control.
Another related art technique described in Marchman (U.S. Pat. No. 5,656,182) utilizes feedback control, however, Marchman does not address attainment of the optimum CD, given tool configuration, calibration, and specification values. Rather it merely performs stage position control as a function of the latent image produced in the substrate.
A further related art technique described in Tadayoshi (JP 6294625 A) discloses a laser beam microscope means for determining the dimensions and profile of a resist pattern but does not disclose analysis of the profile information for z focus control of a photolithographic exposure tool.
Similarly, a related art technique described in Atsushi (JP 11186132 A) discloses resist pattern width measurements to determine an allowable exposure dose range. This disclosure, however does not address the issue of using 3 dimensional resist measurements to feedback a focus bias or to differentiate between a lithography tool focus shift and other process factors.
While the technique of using feedback control using 2 dimensional metrology to overcome a shift in critical dimension due to exposure variation is known in the art, there remains a need in the art for the capability to compensate for tool focus variation directly on product.
It would thus be highly desirable to provide a system and method for independently controlling the variation in optimal product focus offset due to instability in the photolithographic exposure tool, thus obviating the above-mentioned drawbacks of related art techniques.
It would be further highly desirable to provide a system and method of compensating for variation in optimal product focus offset, i.e., correcting focus errors, of a photolithographic exposure tool implementing a production or similar reticle, using a production wafer, and particularly, a system and method that provides CD control by successfully predicting z focus and x and y tilt, while isolating the focus control in the exposure tool from other factors.